FOSSILS OK THE CARBONIFEROUS LIMESTONES. 299 



in two narrow pieces. False hinge area narrow. Surface marked by 

 lii — 21 large elevated rounded radiating plaits, cadi one of which 

 bcreases in size from the umbo to the margin. They also increase 

 in size from the sides to the sinus or fold, on the side of which the 

 largest one occurs. The sinus is occupied by 3 — 4 small plaits of equal 

 size. Length, 5- 16th inch; breadth, 9- 16th inch; thickness, 4-16th 

 inch. 



The above is believed to be identical with a species referred to by 

 Mr Davidson, as represented in the collections submitted to him,. by 

 only one imperfect specimen, and delineated at fig. 15 of his paper. 



Rhynchonella Evangelina, Bartt. — Shell minute, length about 5-1 6 ths 

 of an inch, length and breadth about equal, greatest breadth at middle. 

 In dorsal aspect square, the umbo forming an angle, the three others 

 rounded, in umbonal aspect triangular. Dorsal valve very much 

 larger, and more inflated than the ventral ; -would be semi-globose but 

 for a prominent mesial fold, which originates at or about the middle of 

 the valve, and seems growing higher and higher to the posterior mar- 

 gin, beyond which it projects, forming one of the angles of the square. 

 In profile the fold describes a more or less regular quadrant, the pos- 

 terior part of which becomes parallel, or nearly so, with the longi- 

 tudinal axis of the shell. The summit of the fold bears two or three 

 angular plaits, separated by a deep groove or grooves, which origi- 

 nate with the fold, and grow deeper as the latter becomes more elevated. 

 Fold with steep flattened sloping sides; occasionally these bear 

 minor supplementary grooves. On each side of the shell are two or 

 three large radiating angular plaits, originating about half-way from 

 the umbo to the margin, in running towards which they increase in 

 size, describing in profile a regular curve. As the valve has a more 

 or less steep slope from the fold to the sides of the valve, these 

 plaits have a narrow slope on the inner and a long one on the outer 

 side, so as to give to them an imbricated appearance. Ventral valve 

 depressed forward, with a deep, wide mesial sinus, beginning near 

 the umbo, and corresponding to the mesial fold of the opposite valve. 

 Sinus occupied by prominent plaits corresponding to the grooves on 

 th'' summit of the fold. On the sides of the shell, plaits, corresponding 

 in like manner to these grooves of the opposite valve. Where there are 

 two grooves on the fold, the two corresponding ventral plaits are 

 formed near the anterior part of the sinus by the bifurcation of a single 

 plait. Umbo angular, sharp-pointed, and quite strongly recurved. 

 amen narrow, triangular, with deltidium in two pieces. Interior 

 ol dorsal valve with the two short curved processes so characteristic 

 of the apophysary system of lthynchoiiclla. 



